Typical prior art deflection systems in common use in television receivers include a fixed duty cycle horizontal oscillator coupled by a driver transistor and driver transformer to a horizontal output transistor. The horizontal output transistor is connected to a horizontal deflection winding contained in a yoke associated with a cathode ray tube. The objective of this circuitry is to deflect the electron beam or beams in the cathode ray tube in the horizontal direction at a relatively high rate such as 15,734 lines per second as is used in the NTSC system.
The design of the driver and horizontal output transistor stages is very critical. In typically prior art circuits the horizontal output transistor conducts during the last half of the trace interval and is sharply cut-off to initiate retrace. During retrace a large retrace or flyback voltage appears at the collector of the horizontal output transistor. If the horizontal output transistor is conducting when this voltage appears, the retrace current will be altered and the transistor will dissipate excessive power which generally will result in damage to the transistor. Since the horizontal output transistor is conducting heavily immediately before retrace is initiated, the stored charge in the base and collector regions of the transistor must be removed before the transistor will cease conducting collector current. Normally this stored charge is removed by applying a negative pulse to the base of an NPN transistor to cause a reverse base current to flow. This negative pulse, however, should be applied only as long as necessary to alleviate the voltage stress on the collector-base junction during retrace. R. J. Walker and R. Yu, "Horizontal Output Transistor Base Circuit Design," IEEE Transactions On Broadcast and Television Receivers, Vol. BRT-20, August 1974, pp. 185-192, describe the various design requirements for the base circuit of the horizontal output transistor.
The above design requirements place severe limitations on the horizontal driver transistor and transformer design. In typical circuits conduction by the driver transistor during retrace and the first half of trace switches the output transistor off. The driver transformer stores energy during conduction by the driver transistor which is used to provide forward base current to the horizontal output transistor during the following conduction interval of the output transistor. Accordingly, the drive current in the primary winding must be of the proper amplitude and duration to maintain the horizontal output transistor in conduction in saturation during the second half of trace. When the driver transistor turns on again, the initial current flow through the primary winding of the driver transformer provides a turn-off pulse to the base of the output transistor. The amplitude and duration of the turn-off pulse are dependent on the inductances and the turns ratio of the driver transformer windings.
The critical design requirements of the driver and output stages also necessitates a well-regulated voltage supply for both the driver circuitry and the output circuitry. Accordingly, typical prior art horizontal deflection systems are energized by a voltage supply derived from rectification of the line voltage, but regulated so that the voltage levels vary within narrow limits. Typical prior art voltage regulators used for this purpose include series regulators and constant voltage power transformers as well as other similar circuitry.
One form of known voltage regulator is a switching regulator which regulates by varying the conduction time of a switch with the amplitude of the unregulated voltage. In horizontal deflection systems the horizontal output transistor is operated as a switch. Since the horizontal output transistor does not conduct during the entire trace interval, the portion that it does not conduct deflection current can be used for regulation. P. L. Wessel, "A New Horizontal Output Deflection Circuit," IEEE Transactions on Broadcast and Television Receivers, Vol. BTR-18, August 1972, pp 177-182, and M. J. Maytum, "Transistorized Self-Stabilizing Horizontal-Deflection Systems," IEEE Transactions On Broadcast and Television Receivers, Vol. BTR-20, February 1974, pp. 32-64, describe various self-regulating horizontal deflection circuits. In such circuits the horizontal output transistor is isolated from the deflection winding during that portion of trace when the damper or trace diode is conducting. During this time, the output of the horizontal oscillator is pulse width modulated so that the conduction time of the horizontal output transistor varies over relatively broad limits sufficient to provide regulation of an unregulated voltage supply and to compensate for load variations.
In prior art fixed duty cycle deflection systems, the base drive provided by the driver transformer to the horizontal output transistor is initially large and decays exponentially to a magnitude sufficient to maintain the horizontal output transistor in saturation at the end of trace. Accordingly, excessive base drive is provided throughout most of the conduction interval of the horizontal output transistor thereby dissipating excessive power and generating excessive heat. In self-regulating horizontal deflection systems the drive requirements are even more severe because the conduction and non-conduction intervals vary. Thus, the driver transistor and driver transformer must provide sufficient base drive to cause the horizontal output transistor to conduct in saturation for its longest potential conduction interval. Using typical prior art techniques, however, the base drive would be greatly in excess of that required when the horizontal output transistor is conducting for its minimum conduction interval. Accordingly, in known prior art transformer coupled drivers in both fixed duty cycle and self-regulating horizontal deflection systems the driver circuitry provides excessive base drive to the horizontal output transistor thereby dissipating excessive power and generating excessive heat.